In recent years, with advances in high resolution of sound sources in the audio field, a DSD scheme has attracted attention as a high quality audio data encoding scheme. The DSD scheme has been conventionally employed in SACD (Super Audio CD). However, as the DSD scheme is employed for network delivery of audio data, a high quality sound reproduction system for the DSD scheme is required.
The DSD scheme, which is a kind of PDM (Pulse Density Modulation), can convert an audio waveform into a bit stream through one-bit pulse density modulation and, in principle, can reproduce the original audio waveform by passing the bit stream through a low pass filter.
FIG. 1 is a block diagram of an audio system 100 that reproduces DSD data. Audio data encoded with a DSD scheme are stored in a storage 102. The storage 102 may be a PC or a USB memory. A DSD decoder 200 reads the audio data stored in the storage 102 for each predetermined unit (one sector). The DSD decoder 200 converts the read audio data S1 into audio data S2 of a bit stream format and outputs the audio data S2. A D/A converter 300 converts the audio data S2 of the bit stream format into an analog audio signal S3. An amplifier 104 amplifies the analog audio signal S3 and drives a speaker 106.
A mute function is implemented to prevent a noise from being output from the speaker 106 in a silent state between songs. The present inventors have examined the two following schemes for implementation of the mute function.
The first scheme is to connect the DSD decoder 200 with the D/A converter 300 via a control signal line 108 and switch an electrical state (e.g., a signal level) of the control signal line 108 between a mute state and a non-mute state. The D/A converter 300 reporting the mute state fixes its output S3 such that no noise is output from the speaker 106. However, this scheme has a problem in that the number of pins of the DSD decoder 200 and the D/A converter 300 may be increased or the signal line 108 for mute control is necessary in addition to a line for transfer of the bit stream data S2.
The second scheme is to generate the bit stream data S2 having a predetermined pattern (referred to as a silent pattern) from the DSD decoder 200 in the mute state. A detector that detects the silent pattern is incorporated in the D/A converter 300. The detector determines that it is in a mute state if the silent pattern is repeated several times.
The second scheme may have a problem of noise radiation since the same pattern (bit string) is repeated in the mute state. In addition, if a vendor of the DSD decoder 200 and a vendor of the D/A converter 300 are different from each other, there is a problem in that a mute function does not work when there is a mismatch between a silent pattern output by the DSD decoder 200 and a silent pattern expected by the D/A converter 300.